Gellan is an exocellular microbial polysaccharide, produced from Pseudomonas elodea cell lines and composed of repeating tetrasaccharide units of the following structure:
-3)-.beta.-D-glcp-(1-4)-.beta.-D-glcpA-(1-4)-.beta.-D-glcp-(1-4).alpha.-L-r hamp-(1
wherein glcp corresponds to glucose; glcpA corresponds to D-glucuronic acid; and rhamp corresponds to rhamnose. In its natural form, gellan contains an O-acetyl group in position C(6) of the first glcp residue and O-glyceric groups in position C(2) of the same residue. Natural gellan forms viscous solutions which can give fragile and thermolabile gels after salt addition, especially salts of bivalent cations such as Ca2+ and Mg2+.
Deacylation of natural gellan gives a better gelling agent ("Gerlite"), currently sold for formulations in the alimentary field. In the presence of aqueous solutions of MgCl.sub.2 or CaCl.sub.2 (approximately 0.1% w/v in salt and 0.8-1% w/v in polysaccharide) Gerlite forms highly resistant gels, which are stable when treated in an autoclave and are thermoreversible. They are chemically inert and generally resistant to enzymatic degradation. Gellan is used industrially for its biocompatibility and for the above mentioned special properties. The latter, moreover, according to the results of detailed physico-chemical investigations, markedly depend on the charge density and hydration of the polysaccharide chains as well as on the nature of the counterions. It is therefore to be expected that alteration of the above said critical parameters by partial esterification of the carboxylic groups along gellan chains will markedly influence the physico-chemical behavior of the derivatives (esters) both in bulk and in solution. Similarly, it is to be expected that nature and extent of the interactions between said derivatives and various synthetic and natural species, including proteins, will be profoundly influenced primarily by the chemical nature of the ester moieties and the degree of esterification.
For the preparation of gellan esters with industrial potential, it is important that the esterification procedure be mild, avoiding as much chain degradation as possible, and easily controllable in terms of both the final degree of esterification and yield.
A simple and convenient procedure fulfilling the above requirements encompassed by the present invention, has now been discovered for the preparation of gellan esters based on the treatment of quaternary ammonium salts of gellan with conventional alkylating agents in organic solvents, preferably aprotic, such as methylsulfoxide. The procedure allows the obtainment of a large variety of gellan esters, including mixed esters, above all the esters of monovalent n-aliphatic alcohols and the esters of arylaliphatic, alicyclic and heterocyclic monovalent alcohols.
The procedure also allows the esterification of gellan with appropriate divalent alcohols yielding, in function of the experimental conditions, insoluble derivatives cross-linked to different extents.
The central object of the invention is, however, the new derivatives of gellan in which the uronate residues along the polysaccharide chains are esterified with monovalent alcohols, derivatives which are soluble in water or in organic solvents, and the procedure for their preparation.